Refractory metal roughness reduction using high temperature anneal in hydrides or organo-silane ambients

ABSTRACT

An embodiment of the present invention teaches a method used in a semiconductor fabrication process to form a memory cell in a semiconductor device comprising the steps of: subjecting a layered structure comprising a silicon gate insulating layer, a conductively doped polysilicon gate layer and a refractory metal silicide gate film to a thermal processing step; forming a sheet resistance capping layer directly on the refractory metal silicide film during at least a period of time of the thermal processing step, the sheet resistance capping layer forming a substantially uniform surface on the refractory metal silicide film; patterning and etching the layered structure to form the transistor gate; forming source and drain regions aligned to apposing sides of the transistor gate and formed into an underlying silicon substrate; and forming a storage capacitor (such as a stacked capacitor or a container cell) connecting to one of the source and drain regions. The thermal processing step is performed in a variety of ambients, such as hydrides oxygen/ozone ambients, for a first portion of the time period. In addition, an organo-silane ambient in the later half of the thermal cycle may also be used

This application is a divisional to U.S. patent application Ser. No.08/649,902, filed May 15, 1996 is now U.S. Pat. No. 6,028,002.

FIELD OF THE INVENTION

This invention relates to semiconductor technology, and morespecifically, to a method to reduce refractory metal roughness.

BACKGROUND OF THE INVENTION

In the fabrication of semiconductor wafers, the photolithography stepsare critical when patterning the minimum feature width, dictated bygiven photolithography equipment, onto a wafer. Several factor come intoplay that will affect the dimension and profile of a structure that hasresulted from the photolithography steps performed.

One factor is the quality of the masking material (photoresist) itself.Another factor is the effectiveness of the light source (usuallyultraviolet light) to expose the photoresist in direct correlation to anoverlying mask or reticle. Though, the photoresist quality mustcontinually be monitored and improvements made, the exposure of thephotoresist to a light source to provide the desired patterned, is anarea where major engineering efforts are ongoing.

The effectiveness of proper light exposure of the photoresist, not onlydepends on the photoresist material itself, but also on other factorssuch as the type of underlying film that is being patterned. Maintaininga desired profile becomes even more difficult when patterning a materialhaving a rough surface, such as a refractory metal that is made rough bythe shape of its grains.

When patterning a refractory metal, the unevenness of the grains resultsin the film possessing non-uniform adsorption and reflective propertiesto light. During a photo step, these non-uniform properties to lightresult in the light reflecting back into the photoresist at varyingangles to cause reflective notching of the photoresist. Thoughreflective notching can be caused by any underlying film that is beingpatterned, it is a major problem when patterning the rough surfacedrefractory metal.

Another challenge that is presented by the uneven grain of a refractorymetal presents, comes to play during the etching step. Usually it isdesired to obtain the most vertical profile as possible. However, theuneven grains of the refractory metal silicide, cause the verticalprofile to become jagged and less vertical, both undesirable results.

What is needed is a method that addresses the patterning and etching ofa refractory metal to obtain structures having a substantially verticalprofile. The present invention teaches such a method that may be used insemiconductor processes, such as fabrication processes for DynamicRandom Access Memory (DRAM), Static Random Access Memory (SRAM) and thelike.

SUMMARY OF THE INVENTION

In general, the present invention includes a method for forming arefractory metal structure or a refractory metal silicided structurehaving substantially uniform reflective light properties. In onepreferred implementation, the method includes; subjecting a refractorymetal silicided layer to a thermal processing step; and of forming alayer, having relatively uniform reflective characteristics toultraviolet light, directly on the refractory metal silicided layerduring the thermal processing step. Preferably, the thermal processingstep will be selected to adjust the grain size of the refractory metalto the desired size. The layer formed over the refractory metal caneither be formed during the thermal processing step, or can be formedsubsequently. In many applications, after the thermal processing stepand the formation of the cap layer, the two layers, and possibly otherunderlying layers, may be patterned to form selected features.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A and 1B graphically depict thermal processing cycles used withrespect to the process steps of the present invention;

FIG. 2 depicts an in-process wafer assembly having exemplary transistorgate stack layers formed thereon, illustrated in a verticalcross-section;

FIG. 3 depicts the in-process wafer assembly of FIG. 2 after atransistor gate stack structure is exposed to a photolithographypatterning step and to an etching step; and

FIG. 4 depicts the in-process wafer assembly of FIG. 3 after processsteps have been performed to form source/drain regions to the transistorgate stack and a storage capacitor connected to one of the source/drainregions of the access transistor.

DETAILED DESCRIPTION OF THE INVENTION

Thermal processing steps for forming a refractory metal silicidedstructure of the present invention are demonstrated in FIGS. 1A and 1B.

FIGS. 2-4 depict exemplary embodiments of the present invention whenapplied to specific applications.

Referring now to FIG. 2, substrate 20 (such as silicon or othersemiconductor substrates) has substrate region 26 separated by fieldoxide regions 21. Oxide insulative layer 22 is formed conformally oversubstrate region 26 and field oxide regions 21. A conformal,conductively-doped, polysilicon layer 23 is formed over insulator layer22 and a conformal refractory metal silicide layer 24 is formed overconductive layer 23.

In a preferred embodiment, it is desired that the refractory metalsilicide to be formed is either titanium silicide or tungsten silicide.If tungsten silicide is selected, then in preparation of tungstensilicide deposition, the surface of polysilicon layer 23 is cleaned withhydrofluoric acid to remove any native oxide to enhance adhesion betweenpolysilicon layer 23 and the subsequently deposited tungsten silicidelayer (which will become refractory metal silicide layer 24). If thestructure is to be used as a transistor gate, it is preferred thatapproximately 1200 angstroms of tungsten silicide be deposited onpolysilicon layer 23. During deposition, silane gas is reacted withtungsten hexafluoride in a deposition chamber at 410° C. to form thetungsten silicide. The low resistivity of the deposited tungstensilicide enhances the operating speed of the transistor.

The stacked structure of FIG. 2, which includes layers 22, 23 and 24, issubjected to the thermal processing cycle of either FIGS. 1A and 1B.Taking the case when the stacked structure of FIG. 2 is subjected to thethermal processing cycle of FIG. 1A, during period 1 the structure isannealed at a desired temperature that will adjust the grain size ofrefractory metal silicide layer 24, which in turn sets the resistivityof (sheet resistance) of layer 24. During this same thermal cycle,during period 2, a capping layer 25 (in FIG. 2) is conformally depositedover layer 24. The advantage and importance of this capping layer 25will become apparent as the method is further described.

As show in FIG. 1B, the thermal cycle may be adjusted so that theannealing period 3 occurs at a higher temperature than the temperatureused to deposit capping layer 25, during period 4. It is important thatthe annealing period occurs before the deposition period and at a highenough temperature in order to adjust the refractory metal grain size inorder to set the resistance of the refractory metal silicided structure.The second period may overlap the first period, however, it is preferredthat the deposition of the capping layer be at least a major portion ofthe later half of the entire thermal cycle and it is still furtherpreferred that the deposition of the capping layer comprises a majorportion of the entire thermal cycle.

The thermal processing cycle may be performed in a rapid thermal processsystem, in plasma annealing or in a finance, and at a preferredoperating temperature ranging from 600-1050° C.

During the deposition period several ambients may be employed, such asorgano-silane ambient or a hydride ambient to form a capping material ofsilicon, silicon oxide or silicon nitride, as desired. Also, doped glasslayers, including BPSG, PSG, BSG, etc., may be used. However, it ispreferred that capping material be a doped polysilicon.

A preferred way to form the doped polysilicon comprises feedingorgano-silane into a deposition chamber with the deposition temperatureset at between 450-800° C. Along with the organo-silane, phosphorusdopants are also fed into the chamber. The organo-silane reacts with theheated refractory metal silicide to form an insitu-doped polysiliconcapping layer. Though it is preferred, the polysilicon layer need not beinsitu doped, as it may be doped after the polysilicon is formed. Byselecting polysilicon as the capping layer the resistance of thesilicide structure is further reduced, which would be advantageous ifthis structure is used as a transistor gate.

Referring now to FIG. 3, the stacked structure, now comprising layers22, 23, 24 and 25, is patterned and etched to form metal silicidedstructure 31. As mentioned, the presence of capping layer provides somedefinite advantages when constructing metal silicided structure 31.

One advantage is that capping layer 25 will smooth the surface of metalsilicide layer 24, which is inherently rough due to the graininess ofrefractory metal. Another advantage of capping layer 25 is that it willcancel much of the reflective properties of the refractory metalsilicide. Both of these inherent light properties of layer 25, surfacesmoothness and reduced reflectivity to the electromagnetic spectrum oflight (such as ultraviolet), become a enhancing aid when patterning andetching metal silicided structure 31.

First, during patterning, when a photomask is formed by exposingphotoresist to ultraviolet light a more precise photomask pattern isobtained as capping layer 25, having a smooth surface, will reduce thelight that is reflected back into the photoresist (reducing reflectivenotching of the photoresist) and thus maintaining the desired photomaskpattern.

Second, during the etching of structure 31, because the graininess ofthe refractory metal silicide layer 24 is now capped with thesubstantially smooth surface of layer 25 (smooth with respect to layer24), the subsequent etch will provide sidewall profiles (orsubstantially uniform etch profiles) that are more vertically uniformthan can be acquired when etching several layers that are covered withrefractory metal silicide alone.

For example, during a plasma etch, the etch continues in a substantiallyanisotropic direction until completed, because layer 25 has uniformlight absorption and minimum light interference properties, incomparison to those properties inherent in refractory metal silicidelayer 24. The process to form structure 31 may be utilized in anyfabrication process that employs a refractory metal silicide structure.

For example, the present invention lends itself to a method ofpatterning a transistor gate in a semiconductor device. First, a layeredstructure comprising a silicon gate insulating layer, a conductivelydoped polysilicon gate layer and a refractory metal silicide gate film,is subjected to a thermal processing step. Next, a sheet resistancecapping layer is formed directly on the refractory metal silicide filmduring at least a period of time of the thermal processing step. Thesheet resistance capping layer forms a substantially uniform surface onthe refractory metal silicide film and after, patterning and etchingsteps, the resulting patterned, layered, structure forms the transistorgate.

The above process could be expanded to form a memory cell (as depictedin FIG. 4) by forming diffusion region 41, forming storage plate 42connecting to a diffusion region 41 (source and drain regions of atransistor), covering storage plate 42 with capacitor dielectric 43 andforming the top capacitor plate 44 to complete the memory cell. Theprocess may be used for a stacked capacitor process or a container cellcapacitor process. Furthermore, though the embodiments are described interms of improving the profile of a refractory metal silicide structureduring patterning and etching, the disclosed patterning and etchingprocess will also enhance the profile of a refractory metal structure.

Although the present invention is described in various embodiments, itwill be apparent to one skilled in the art that changes andmodifications may be made thereto without departing from the spirit andscope of the invention as claimed.

What is claimed is:
 1. A method for forming a refractory metal silicidedstructure having substantially uniform reflectivity, said methodcomprising the steps of: subjecting a refractory metal silicided layerto a thermal processing step; forming a layer having relatively uniformreflective characteristics to light directly on said refractory metalsilicided layer during said thermal processing step.
 2. The method ofclaim 1, wherein said light comprises the electromagnetic lightspectrum.
 3. The method of claim 1, wherein said thermal processing stepcomprises at least a first and second annealing period.
 4. The method ofclaim 3, wherein said first and second anneal periods overlap.
 5. Themethod of claim 3, wherein said second anneal period follows said firstanneal period.
 6. The method of claim 3, wherein said first and secondanneal periods are performed at approximately the same temperature. 7.The method of claim 3, wherein said first and second anneal periods areperformed at different temperatures.
 8. The method of claim 1, whereinsaid thermal processing cycle comprises a process selected from thegroup consisting of rapid thermal process annealing, plasma annealingand furnace annealing.
 9. The method of claim 1, wherein said thermalprocessing cycle comprises an operating temperature range of 600-1050°C.
 10. A method for forming a refractory metal silicided structurehaving substantially uniform etch profiles during semiconductorfabrication, said method comprising the steps of: subjecting arefractory metal silicided layer to a thermal processing step; forming arelatively uniform layer directly on said refractory metal silicidedlayer during said thermal processing step; patterning and etching saidrefractory metal silicided layer to form said refractory metal silicidedstructure.
 11. The method of claim 10, wherein said thermal processingstep comprises a first anneal period for adjusting the grain size ofsaid refractory metal silicide and a second anneal period for formingsaid relatively uniform layer.
 12. The method of claim 11, wherein saidfirst and second anneal periods overlap.
 13. The method of claim 11,wherein said second anneal period follows said first anneal period. 14.The method of claim 11, wherein said first and second anneal periods areperformed at approximately the same temperature.
 15. The method of claim11, wherein said first and second anneal periods are performed atdifferent temperatures.
 16. The method of claim 10, wherein said thermalprocessing cycle comprises a process selected from the group consistingof rapid thermal process annealing, plasma annealing and furnaceannealing.
 17. The method of claim 10, wherein said thermal processingcycle comprises an operating temperature range of 600-1050° C.
 18. Amethod for forming a refractory metal structure having substantiallyuniform reflectivity, said method comprising the steps of: subjecting arefractory metal layer to a thermal processing step; forming a layerhaving relatively uniform reflective characteristics to light directlyon said refractory metal layer during said thermal processing step. 19.The method of claim 18, wherein said light comprises the electromagneticlight spectrum.
 20. The method of claim 19, wherein said thermalprocessing step comprises at least first and second annealing periods.21. The method of claim 20, wherein said first and second anneal periodsoverlap.
 22. The method of claim 20, wherein said second anneal periodfollows said first anneal period.
 23. The method of claim 20, whereinsaid first and second anneal periods are performed at approximately thesame temperature.
 24. The method of claim 20, wherein said first andsecond anneal periods are performed at different temperatures.
 25. Themethod of claim 18, wherein said thermal processing cycle comprises aprocess selected from the group consisting of rapid thermal processannealing, plasma annealing and furnace annealing.
 26. The method ofclaim 18, wherein said thermal processing cycle comprises an operatingtemperature range of 600-1050° C.
 27. The method of claim 18, whereinsaid layer is a material selected from a group consisting of silicon,silicon oxide, silicon nitride, BPSG, PSG, BSG and conductively dopedpolysilicon.